A method for electronically controlling ratio shift dynamics of an automatic transmission during a friction-element-to-friction-element ratio change.
The strategy and control method of the invention is capable of being used in an electronically controlled automatic transmission for automotive vehicle powertrains for transferring torque from an internal combustion engine to vehicle traction wheels. Automatic transmissions commonly used in vehicle powertrains include multiple ratio gearing wherein reaction torque and relative speeds of the gearing elements are controlled by pressure-operated friction elements; i.e., friction clutches and brakes. Managing torque transfer from the engine to the traction wheels is accomplished by an electronic controller that responds to powertrain variables, including engine variables and driver commands.
Engagement and release of the friction elements is accomplished as an electronic controller develops clutch and brake control pressure so that the speed/torque characteristics of the engine is matched to a desired traction wheel torque.
Driver circuits receive control signals from the controller to actuate solenoid valves. One solenoid valve typically would be used to control circuit pressure delivered to pressure-operated servos for the clutches and brakes. The solenoid valves for effecting control of the pressure-operated clutches and brakes, as well as the solenoid valve for pressure control, may be either variable force solenoids or pulse width modulated solenoids.
The output of the solenoid valves is used to control upshifts and downshifts of a shift valve circuit, which in turn control distribution of regulated circuit pressure (i.e., line pressure) to the pressure-operated clutches and brakes. The clutch and brake actuating pressure usually is modified further by intermediate modulator valves or pressure modifier valves to tailor the actual pressure distributed to the pressure-operated clutches and brakes, depending upon the control parameters required by the system.
It is common practice in the case of a multiple ratio powertrain for automotive vehicles to establish nonsynchronous ratio changes wherein one of a pair of friction elements involved in a ratio change is controlled by an overrunning coupling and the companion friction element involved is actuated by a fluid pressure actuator. Such a shift commonly is referred to as a nonsynchronous shift. Only a single friction element is involved. Such conventional control systems, of necessity, are relatively complex because of the necessity for using multiple shiftable valves between the control solenoid valves and the clutches and brakes. Further, the necessity for using overrunning couplings to control relative motion of the gear elements during ratio changes increases the complexity, size and weight of the transmission.
An example of a conventional automatic transmission having shift solenoids that trigger the operation of shift valves in a control valve circuit for pressure-operated clutches and brakes is disclosed in U.S. Pat. Nos. 5,835,875 and 6,122,583, as well as U.S. Pat. No. 5,460,582.
Another example of a conventional transmission control circuit of this kind, which includes circuit pressure control logic, is disclosed in U.S. Pat. Nos. 5,157,608 and 5,305,663. Each of these prior art patents is assigned to the assignee of the present invention.
In the case of the control system of the ""582 patent, the circuit pressure made available to the pressure-operated friction elements is modified further by pressure accumulators, which are needed to tailor the rates of pressure buildup in the actuators for the friction elements to improve ratio shift quality.
It is an objective of the invention to provide a strategy and method for controlling friction elements of a multiple ratio automatic transmission by providing smooth torque transition between friction elements as one friction element is applied and a companion friction element is released during a direct friction-element-to-friction-element ratio shift. It is possible, using the teachings of the invention, to control each friction element independently, one with respect to another, during gear ratio changes. Solenoid valves for controlling the shifts can be individually calibrated to effect optimum shift quality without affecting the characteristics of the solenoid valves that control other shifts. This simplifies the shift calibration and improves the reliability of the calibration.
The method and strategy of the invention is applicable to a ratio shift that involves a clutch-to-clutch shift, or a clutch-to-band shift, or a band-to-clutch shift, or a band-to-band shift.
A system capable of using the improved method and strategy of the invention includes powertrain sensors for developing input signals for an electronic digital microcontroller. The microcontroller stores input information and executes control logic as it reads sensor outputs. A duty cycle control signal, or a variable force solenoid current signal, is transferred to solenoid control valves for the oncoming friction element and the offgoing friction element during a ratio shift.
If the transmission includes a hydrokinetic torque converter with an impeller connected to the engine and a turbine connected to the transmission input shaft, turbine speed would be one of the sensor inputs as well as engine speed and output shaft speed. Other inputs are throttle position, transmission range selector position, transmission oil temperature, a brake signal and an air conditioning on/off signal. The duty cycle or the variable force solenoid current output is calculated for the oncoming and offgoing friction elements based on the control logic.
The system includes driver circuits that develop input signals for shift control solenoid valves, thereby producing a hydraulic pressure at the oncoming friction element. Likewise, the offgoing friction element pressure is controlled as a smooth torque and speed transition occurs during a gear shift.
The control system used in practicing the method and strategy of the invention has a pressure-controlled circuit including solenoid valve actuators that communicate with the friction elements, which in turn control the gearing to establish plural torque flow paths. The signal input portions of the electronic controller communicate with sensors that measure powertrain variables including engine speed, output shaft speed, engine throttle position, and transmission speed ratio range selection. The strategy includes determining the input variables by reading the output of the sensors, computing friction element capacity reduction for the appropriate friction element on a ratio change, maintaining pressure on the friction elements to initiate engagement, increasing the oncoming clutch pressure during a transfer phase as a function of the ratio of friction element torque to input torque and clutch gain, controlling the oncoming and offgoing clutch pressures to effect a calibrated speed ratio rate of change as a function of the throttle position, and controlling the friction elements using a closed-loop control to obtain a speed ratio change rate of a calibrated value.
The strategy and method of the invention further includes entering an incipient capacity phase after the commanded offgoing clutch reaches a target pressure and controlling the offgoing friction element pressure with a closed-loop control in proportion to the gain in capacity of the oncoming clutch capacity. The engagement of the offgoing friction element is initiated during an upshift to make possible a reversal of the upshift sequence before it is completed. The oncoming friction element pressure is boosted to a maximum pressure during a ratio downshift until the speed ratio value is approximately at the next speed ratio for a given amount of time.